The present invention relates to a high speed laser drilling apparatus using a plurality of laser beams.
A printed circuit board used in a mobile telephone or the like has been smaller year after year. Further, the number of holes drilled in a circuit board has been increased, that is, not less than 3,000 of micro-sized holes are formed therein.
In order to form micro-sized holes in a printed circuit board, a micro-hole drilling process using a laser beam is used. As to this process, as disclosed in JP-A-63-147138 (Refer to in particular to FIGS. 1 and 2), a laser drilling process in which two mirrors (which will be hereinbelow referred to as “galvanometer-mirror system”) that are rotated around two orthogonal axes, respectively, and an fθ lens are used for converging light and positioning the converging point, has been prosperously used. As the number of micro-sized holes is increased, there has been increased demands of to increase the drilling speed. It is noted that the fθ lens is the one which is designed so that the relationship between an incident angle θ of a parallel ray beam and a light focusing point y is expressed by the formula y=fθ.
In order to speed up the drilling process, as disclosed in JP-A-2000-190087 (refer to in particular FIG. 1), there has been proposed a laser drilling apparatus in which a laser beam is divided into an S-polarized beam and a P-polarized beam by a polarizing beam splitter, and angles of these beams are determined by galvanometer-mirror systems which are operated independent of each other, and which are placed in the optical paths of these two beams, respectively, and thereafter, they are reflected by and transmitted through a polarized beam mixer (a polarizing beam splitter is conversely used) so that the beams are incident upon the fθ lens in order to simultaneously drill the circuit board at two positions.
There has been known, as prior art to the present invention, a laser drilling machine capable of simultaneously drilling a circuit board at two positions, in which a laser beam is divided into two beams with the use of two acousto-optic elements, as disclosed in JP-A-2000-263271 (Refer to in particular to FIG. 1). In the case of using two galvanometer-mirror systems for one fθ lens, due to the size of the galvanometer-mirror systems, the galvanometer-mirror systems must be placed distant from the fθ lens. Accordingly, since the galvanometer-mirror systems are remote from the focal point of the fθ lens, a deficiency which is the so-called telecentric error is generated. The telecentric error means such a phenomenon that a light beam focused by the fθ lens can not be incident at right angles upon a surface to be worked, that is, it is incident at a certain angle to the normal to the surface, since the light beam which is incident upon the fθ lens does not pass through the focal point of the lens.
On the other hand, in the case of the laser drilling device disclosed in JP-2000-263271, without increasing the number of components including the fθ lens, it has been desired to simultaneously drill a circuit board at more than three positions.